Class MailboxGroup.

The MailboxGroup class models a client's group of mailboxes, including the likelihood that the client actually has such a group.

Many clients like to perform the same operation on many mailboxes. In order to limit load and improve performance, Archiveopteryx tries to detect that and restructure the work done.

An instance of this class is created when Archiveopteryx thinks that such an operation may be starting. When a mailbox operation is performed on something this object contains(), a user can check the number of hits() and if deemed large enough, it may choose to process the remaining contents() in advance and cache the results.